Low temperature chemical vapor deposition of silicon dioxide films

ABSTRACT

A method for low temperature chemical vapor deposition of an SiO 2  based film on a semiconductor structure using selected alkoxysilanes, in particular tetramethoxysilane, trimethoxysilane and triethoxysilane which decompose pyrolytically at lower temperatures than TEOS (tetraethoxysilanes). Ozone is introduced into the reaction chamber to increase deposition rates, lower reaction temperatures and provide a better quality SiO 2  film by generating a more complete oxidation. Ozone is also employed as a reactant for doping SiO 2  based films with oxides of phosphorus and boron.

This application is a continuation of application Ser. No. 06/744,838,filed June 14, 1985 now abandoned.

FIELD OF THE INVENTION

This invention relates to a method for depositing an SiO₂ baseddielectric film on a semiconductor structure and in particular to amethod for low temperature chemical vapor deposition of SiO₂ films usinga selected alkoxysilane.

BACKGROUND OF THE APPLICATION

Tetraethoxysilane (TEOS) has been used extensively as the sourcematerial for the chemical vapor deposition (CVD) of an SiO₂ layer on asemiconductor wafer. U.S. Pat. No. 3,934,060, issued to Burt et al. onJan. 20, 1976, describes on such prior art process.

In a typical prior art process nitrogen is used as the carrier gas whichis passed over or bubbled through the TEOS which is held in a sourcecontainer. The wafers to be covered with a deposited SiO₂ film are laidin a furnace boat in a furnace tube into which the TEOS vapor entrainedin the carrier gas is introduced and the wafers are heated to a selectedtemperature at which the TEOS pyrolytically decomposes. The range oftemperatures less than approximately 960° C. at which TEOS pyrolyticallydecomposes is well known. As the temperature of the wafers is reducedthe decomposition rate and the film deposition rate decrease until theyappear to stop. As a practical matter the range of temperaturestypically employed ranges between 650° C. and 950° C. While the growthrate is lower for temperatures at the lower end of the range, theselower temperatures have the advantage of causing less thermal damage tothe semiconductor circuit embedded in the underlying silicon wafer.These lower temperatures are also more compatible with metalizationschemes using aluminum and its alloys.

U.S. Pat. No. 3,934,060, referred to above, describes an improvement inthe geometry and deposition technique of the typical prior art process,which results in a more uniform coverage of the wafers by the SiO₂ film.

It is also known that the growth rate of the SiO₂ layer can be enhancedby using oxygen as a carrier gas (See U.S. Pat. No. 3,614,584, issued toInoue on Oct. 19, 1971). The Inoue patent also describes a method fordepositing TEOS using tetraethoxysilane as a source together withtriisopropyl titanate in order to deposit a composite insulating layerof TiO₂ -SiO₂ where the TiO₂ constitutes less than 0.02% by weight ofthe layer. Inoue employs the temperature range of 300° C. to 500° C. forthe above process. Other methods using plasma deposition techniques areknown for depositing an SiO₂ based film on a video disk. For U.S. Pat.No. 4,282,268, issued Aug. 4, 1981 to Priestley et al., which isincorporated herein by reference, describes a method for forming an SiO₂layer on a video disk by introducing into an evacuated chamber adielectric precursor having the formula ##STR1## where R₁ is selectedfrom the group H and --CH₃, R₂ and R₃ are independently selected fromthe group consisting of H, --CH₃, --OCH₃ and --OC₂ H₅ and R₄ is selectedfrom the group consisting of --OCH₃ and --OC₂ H₅. The SiO₂ layer isdeposited onto the disk using a glow discharge. This method has provenunsuitable for depositing an SiO₂ based film on a semiconductor waferbecause the deposited film has a poor dielectric quality, low densityand is highly particulate, and because the deposited film has poor stepcoverage.

SUMMARY OF THE INVENTION

In contrast to the prior art use of TEOS for the pyrolytic decompositionof SiO₂ based films on semiconductor structures the present inventionutilizes other selected alkoxysilanes which permit the reactions toproceed at lower temperatures and at higher deposition rates. In oneembodiment, the semiconductor structure is heated to a selectedtemperature less than or equal to 900° C. and an alkoxysilane selectedfrom the group consisting of tetramethoxysilane, a trialkoxysilanehaving the formula ##STR2## where R is a methyl, ethyl or propyl group,a dialkoxysilane having the formula ##STR3## where R is a methyl, ethyl,or propyl group or a vinyltrialkoxysilane having the formula ##STR4##where R is a methyl or ethyl group. Oxygen and/or nitrogen are used as acarrier gas with oxygen being preferred. When the wafer temperature isreduced to a range between 550° C. and 650° C. the alkoxysilane may beselected to be tetramethoxysilane, triethoxysilane or trimethoxysilane.When the reaction temperature is selected to be between 350° C. and 550°C. the alkoxysilane selected may be tetramethoxysilane ortrimethoxysilane.

It has also been discovered that when ozone is introduced into thereaction chamber along with nitrogen or oxygen, with the concentrationof ozone preferably being less than or equal to 10% by weight, thereaction rate is increased by nearly an order of magnitude foralkoxysilanes. Alternatively, this introduction of ozone permit thereaction to proceed at lower temperatures for a given SiO₂ depositionrate. For example, using a trialkoxysilane (including avinylalkoxysilane) or a dialkoxysilane the wafer temperature may beselected to be in the range of 350° C. to 500° C. A temperature in thisrange has distinct advantages in semiconductor processing since it iscompatible with the wafer metalization and there is little or no thermaldamage to the electrical properties of the circuits embedded in thesilicon wafer. It has also been discovered that the use of ozone in thereaction chamber permits the doping of the SiO₂ film with oxides ofphosphorous and boron at temperatures between 350° C. and 900° C. Theuse of ozone in this connection provides an important advantage over theprior art since it greatly reduces the temperatures at which the dopingoxides are formed compared to those required when oxygen alone is usedas a carrier gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reactor suitable for employing the methods of thisinvention.

FIG. 2 shows a reactor similar to FIG. 1 including a second bubblechamber for use in depositing a doped SiO₂ film.

DETAILED DESCRIPTION

FIG. 1 shows a cross-sectional schematic diagram of a microreactor 10for employing the methods of the present invention for chemical vapordeposition of an SiO₂ -based dielectric film on a semiconductorstructure, for example a silicon wafer. Microreactor 10 is described ina copending application of Imad Mahawili entitled "Microreactor," U.S.Ser. No. 745,336, now abandoned, filed with the U.S. Patent andTrademark Office on June 14, 1985. Microreactor 10 includes housing 1surrounding reactor chamber 2. Housing 1 is made of stainless steel orother inert material which does not react with the gases introduced intochamber 2. Heater plate 3, typically a block made of stainless steel, isheld in position in chamber 2 by support rod 5 attached to housing 1.Silicon wafer 4, upon which the SiO₂ -based film is to be deposited, isplaced on heater plate 3. The temperature of plate 3 is regulated bytemperature controller 6 which is connected through thermocouple 7 to acartridge heater embedded in heater plate 3.

Silicon wafer 4 rests upon heater plate 3 and is heated thereby to aselected temperature as determined by temperature controller 6. Reactor10 can be operated in the isothermal or cold-wall modes for a range oftemperatures from room temperature up to 900° C. Exhaust chamber 14 isconnected to a vacuum pump (not shown) so that microreactor 10 can beoperated at reduced pressures, if desired. In the methods of depositingSiO₂ -based films described herein, reactor 10 was operated at pressuresranging from atmospheric pressure down to 0.5 torr.

An alkoxysilane 9 is introduced into bubble chamber 8 at atmosphericpressure and room temperature (at room temperature, alkoxysilanes aregenerally in liquid form). A carrier gas such as nitrogen or oxygen isintroduced via gas line L₁. The flow rate of the carrier gas intoalkoxysilane 9 in bubble chamber 8 is regulated by control valve V₁. Thecarrier gas, together with alkoxysilane vapor, flows from bubble chamber8 via gas line L₂ to the elongated portion 11 of reactor chamber 2 asshown in FIG. 1. The flow rate through line L₂ is controlled by valveV₂. A second selected mixture of one or more gases, for example oxygen,nitrogen, or ozone, may also be introduced into the elongated portion 11of chamber 2 via line L₃, which is regulated by flow rate control valveV₃.

The above chemical vapor deposition reactor was used in order to studythe deposition rates of SiO₂ -based films for selected alkoxysilanes notpreviously used for the chemical vapor deposition of SiO₂ -based filmson semiconductor wafers. Wafer 4 was maintained at various selectedtemperatures for selected carrier gases introduced via line L₁ and forselected gases introduced via line L₃. The deposition rate for the SiO₂-based film using prior art alkoxysilane tetraethoxysilane Si(OC₂ H₅)₄(TEOS) was used as a control process. Absolute deposition rates were notmeasured for the alkoxysilanes tested. However, relative rates weredetermined by visual inspection of the deposited SiO₂ film. The reactorwas operated in both the cold wall mode and the hot wall mode.

    ______________________________________                                        CONTROL PROCESS 1                                                             ______________________________________                                        Alkoxysilane: tetraethoxysilane (TEOS) Si(OC.sub.2 H.sub.5).sub.4,                          electronic grade 99.9% pure;                                                  molecular weight, 208.3 g/mole;                                               boiling point 169° C.                                    Carrier gas via                                                                             Nitrogen                                                        line L.sub.1 :                                                                Gas via       None (line L.sub.3 off)                                         line L.sub.3 :                                                                Flow rate via 20-100 sccm (20, 50, 100) sccm                                  line L.sub.1 :                                                                Flow rate via 0                                                               line L.sub.3                                                                  Wafer temperature:                                                                          650° C.-900° C. (650°, 700°,                      750°,                                                                  900°) ±20° C.                                  Reaction time:                                                                              15-30 minutes (15, 30) minutes                                  Pressure:     1 atmosphere to 0.5 torr                                                      (1 atmosphere, (10, 5, 0.5) torr)                               ______________________________________                                    

Control process 1 was carried out for the specific choices of flowrates, temperatures and pressures indicated in parentheses.

EXAMPLE 1

Same as control process 1 except tetramethoxysilane, Si(OCH₃)₄ having amolecular weight of 152.2 g/mole and a boiling point of 121°-122° C. wasused in place of tetraethoxysilane.

EXAMPLE 2

Same as control process 1 except triethoxysilane H-Si(OC₂ H₅)₃, having amolecular weight of 164.3 g/mole and a boiling point of 131.1° C. wasused in place of tetraethoxysilane.

EXAMPLE 3

Same as control process 1 except that trimethoxysilane, H-Si(OCH₃)₃, 95%pure, having a molecular weight of 122.2 and a boiling point of 86°-87°C. was used in place of tetraethoxysilane.

Control process 1 and examples 1 through 3 all yielded SiO₂ films withthe following relative deposition rates, r:r(tetraethoxysilane)<r(triethoxysilane)<r(tetramethoxysilane)<r(trimethoxysilane).

CONTROL PROCESS 2

Same as control process 1, except that 99.999% pure 02 was used as thecarrier gas via line 1 in place of nitrogen. Again control process 2 wascarried out for the specific choices of flow rates, temperatures andpressures within the ranges stated as indicated by the values inparentheses.

EXAMPLE 4

Same as control process 2, except that tetramethoxysilane was used inplace of tetraethoxysilane.

EXAMPLE 5

Same as control process 2, except that triethoxysilane was used in placeof tetraethoxysilane.

EXAMPLE 6

Same as control process 2, except that trimethoxysilane was used inplace of tetraethoxysilane.

Control process 2 and examples 4 through 6 all yield SiO₂ films with thefollowing relative deposition rates:r(tetraethoxysilane)<r(triethoxysilane)<r(tetramethoxysilane)<r(trimethoxysilane).

An examination of the above processes and examples showed thatcorresponding reaction rates were increased when oxygen was used as acarrier gas in place of nitrogen.

CONTROL PROCESS 3

Same as control process 2, except that the wafer temperature was550°-650° C. (550° C., 600° C., 650° C.) ±20° C. were the actual wafertemperatures employed.

Examples 7, 8 and 9 are the same as examples 4, 5 and 6, except that thewafer temperature was a selected temperature in the range 550°-650° C.(550° C., 600° C., 650° C.) ±20° C. were the wafer temperaturesselected. The deposition rates in this lower temperature range wereordered in the same way as given for examples 4, 5 and 6; however, itwas noted that very little SiO₂ film was deposited for TEOS(tetraethoxysilane).

CONTROL PROCESS 4

Same as control process 2, except that wafer temperature was reduced toa selected temperature in the range 450°-550° C. Examples 10, 11 and 12are the same as examples 4, 5 and 6, except that the wafer temperaturewas a selected temperature in the range 450° C. to 550° C. Thetemperatures selected were 450° C., 500° C., and 550° C. ±20° C. Atthese lower temperatures, no SiO₂ film was formed using the controlprocess 4 or the example process number 11. An SiO₂ film was formed inexamples 10 and 12 with the SiO₂ deposition rater(tetramethoxysilane)<r(trimethoxysilane).

CONTROL PROCESS 5

Same as control process 2 except that the wafer temperature was aselected temperature in the range 350°-450° C. The selected temperatureswere 350° C., 400° C. and 450° C. ±20° C.

Examples 13, 14, and 15 are the same as examples 4, 5, and 6 except thatthe wafer temperature was in the range 350°-450° C. At these reducedtemperatures, no SiO₂ film was formed using the method of controlprocess 5 or example 14. A very thin film formed using the process ofexample 13. A thicker film was formed using the process of example 15,i.e., r(tetramethoxysilane)<r(trimethoxysilane).

The above examples for semiconductor applications demonstrate that anSiO₂ film of good dielectric quality can be deposited from decompositionof various alkoxysilanes other than TEOS. The use of tetramethoxysilane,triethoxysilane and trimethoxysilane permits lower wafer temperatures(≦650° C.) than prior art TEOS, which is deposited at temperaturesbetween 650° C. and 900° C.

In particular, a good quality SiO₂ based film is deposited usingtetramethoxysilane or trimethoxysilane at wafer temperature as low as350°-550° C., good quality SiO₂ based film is deposited usingtetramethoxysilane, trimethoxysilane, and triethoxysilane at wafertemperatures of 550° C. to 650° C. Moreover, in the temperature range550° to 650° C., the deposition rate of the SiO₂ based film was greaterusing tetramethoxysilane, trimethoxysilane and triethoxysilane than thedeposition rate using TEOS.

Depositing an SiO₂ based film on an IC silicon wafer using these lowerreaction temperatures has an important advantage over the highertemperature reaction required when using TEOS: the electrical propertiesof the integrated circuit wafer are less affected by lower temperatureprocessing. For example, lower temperature processing causes lessdiffusion of dopants in the underlying integrated circuit and is morecompatible with aluminum metalization.

The use of the methoxysilanes described above has the added advantagethat they reduce the amount of residual carbon in the deposited SiO₂based film, which enhances the dielectric quality of the film.

In Examples 1 through 15, above, the alkoxysilanes used weretetramethoxysilane, triethoxysilane or trimethoxysilane.

However, in general the following alkoxysilanes may be employed with themethod of this invention:

(1) a trialoxysilane having the formula ##STR5## where R=a methyl (CH₃),ethyl (C₂ H₅), or propyl (C₃ H₇) group

(2) a dialkoxysilane having the formula ##STR6## where R=a methyl (CH₃),ethyl (C₂ H₅), or propyl (C₃ H₇) group

(3) a tetramethoxysilane ##STR7## where R=methyl (CH₃), or

(4) a vinyltrialkoxysilane having the formula ##STR8## where R=a methyl(CH₃), or ethyl (C₂ H₅) group

It has also been discovered that important advantages can be obtainedusing ozone as a reactor gas, in particular:

(1) for selected alkoxysilanes, such as trialkoxysilanes,vinyltrialkoxysilanes, and dialkoxysilanes, the wafer temperature may bereduced to the range of 300°-500° C., a temperature which does notadversely affect the properties of active and passive devices formed inand on the wafer surface;

(2) for the temperature range 300°-900° C. a better quality SiO₂dielectric film having lower residual carbon is obtained due to a morecomplete oxidation reaction;

(3) the deposition rate of the SiO₂ dielectric film is increased; and

(4) doping of the SiO₂ film with both phosphorous and boron oxides isreadily achievable.

Example 16, below, provides one example of the use of ozone in thechemical vapor deposition of SiO₂ based films.

In example 16, where ozone is employed as a reactant gas, one may useany alkoxysilane including tetraethoxysilane (TEOS).

EXAMPLE 16

    ______________________________________                                        Alkoxysilane:  tetraethoxysilane,                                                            tetramethoxysilane,                                                           triethoxysilane, or                                                           trimethoxysilane                                               Carrier gas via                                                                              O.sub.2 or Nitrogen                                            line L.sub.1:                                                                 Reactant gas via                                                                             O.sub.2 and less than 10% ozone by                             line L.sub.3 : weight                                                         Flow rate for carrier                                                                        20-100 sccm (20, 50, 100) sccm                                 gas via line L.sub.1 :                                                        Flow rate for gas                                                                            20-200 sccm (20, 50, 100,                                      via line L.sub.3 :                                                                           200) sccm                                                      Wafer temperature:                                                                           300-900° C. (300° C., 350° C.,                           - 400° C., 500° C., 600° C.,                             700° C., - 800° C., 900° C.)                             ±20° C.                                              Pressure:      1 atmosphere to 0.5 torr                                                      (1 atmosphere, (10, 5, 0.5)                                                   torr)                                                          ______________________________________                                         (The values of the parameters in parenthesis indicate the values tested) 

Example 16 may be modified by using a mixture of O₂ and nitrogen for thecarrier gas via line L₁ or by using a mixture of O₂, nitrogen and lessthan 10% ozone by weight for the reactant gas via line L₃.

In another embodiment of the invention example 16 was modified by usinga mixture of O₂ and less than 10% ozone modified by using a mixture ofO₂ and ozone, where the weight percent of ozone present is greater than0 percent but less than substantially 10 percent it is preferable tointroduce ozone via line L₃.

An ozone generator (not shown) containing a standard electrodelessdischarge tube and capable of producing a concentration of 10% ozone byweight or less from a feed stream of pure oxygen is attached to line L₃.

FIG. 2 shows a reactor suitable for producing a doped SiO₂ film. FIG. 2is similar to FIG. 1 except that a second bubble chamber 15 is providedwhich contains a dopant source, for example trimethylphosphite ortrimethylborate. A carrier gas such as O₂ or N₂ is introduced into thedopant source via line L₄, which is controlled by valve V4. The dopedgas passes via control valve V₅ and line L₅ into mixing chamber 11.

EXAMPLE 17 (not tested)

    ______________________________________                                        Alkoxysilane:    any alkoxysilane                                             Carrier gas via Line L.sub.1 :                                                                 O.sub.2 or Nitrogen                                          Reactant gas via Line L.sub.3 :                                                                O.sub.2 and less than 10% ozone                                               by weight                                                    Carrier gas via line L.sub.4 :                                                                 O.sub.2 or Nitrogen                                          Flow rate via line L.sub.1 :                                                                   20-100 sccm                                                  Flow rate via line L.sub.3 :                                                                   20-200 sccm                                                  Flow rate via line L.sub.4 :                                                                   20-100 sccm                                                  Wafer temperature:                                                                             300°-900° C.                                   Pressure:        1 atmosphere to 0.5 torr                                     Dopant:          trimethylphosphite P(OCH.sub.3).sub.3                        ______________________________________                                    

The advantage of using ozone in the gas stream is that ozone readilyoxidizes both phosphorous and boron hydrides and alkoxides which areused as dopant sources for doping SiO₂ films. Moreover, oxidation in thepresence of ozone takes place at substantially lower temperatures thanwhen pure O₂ is used as the carrier gas. Thus, while the temperaturerange shown in example 17 is 300°-900° C., the preferred temperaturerange is lower for selected alkoxysilanes. For example, for atrialkoxysilane the preferred temperature range is 300°-500° C., whichdoes not damage the underlying semiconductor structure.

The above examples are meant to be exemplary and not limiting and inview of the above disclosure many modifications will be obvious to oneof ordinary skill in the art without departing from the scope of theinvention.

In particular, the precise configuration of the deposition reactor isnot critical to using the methods of this invention and one of ordinaryskill in the art will be able to employ other suitable reactors in viewof the disclosures made herein.

I claim:
 1. A methyl of forming a SiO₂ based layer by chemical vapordeposition on a semiconductor structure, the method comprising the stepsof:introducing the semiconductor substrate into a reactor chamber;heating the semiconductor structure to a predetermined temperature inthe range of substantially 300° C.-500° C.; introducing a predeterminedgas reactant into the reactor chamber, with the first gas reactant beingselected from a trialkoxysilane having the formula ##STR9## where R is amethod group, and ethyl gruop, or a propyl group; introducing apredetermined oxidizing gas reactant, consisting essentially of amixture of oxygen and ozone, where the weight percentage of ozonepresent in the oxidizing gas reactant is greater than 0 percent but lessthan substantially 10 percent, into the reactor chamber, where thereactor chamber pressure is between substantially 0.5 Torr and oneatmosphere; allowing the gas reactant and oxidizing gas reactant tothermally react to produce molecules of SiO₂ ; and allowing the SiO₂molecules thus produced to be deposited on the semiconductor structure.